1. Technical Field
Embodiments described in the present disclosure are directed generally to catalytic heaters and heaters for warming storage tanks containing fluids that are normally gaseous at normal atmospheric pressure and typical ambient temperatures, and in particular to catalytic heaters configured to be coupled to such storage tanks, and including pilot heaters to enable rapid activation of the heaters.
2. Description of the Related Art
A number of fluids that are normally found in gaseous form are commonly stored and transported under pressure as liquids, including, for example, methane, butane, propane, butadiene, propylene, and anhydrous ammonia. Additionally, fuel gasses comprising one or more constituent gasses are also stored and transported under pressure as liquids, including, e.g., liquefied petroleum gas (LPG), liquefied natural gas (LNG), and synthetic natural gas (SNG). Of these, LPG is perhaps the most commonly used. Accordingly, the discussion that follows, and the embodiments described, refer specifically to LPG. Nevertheless, it will be understood that the principles disclosed with reference to embodiments for use with LPG tanks can be similarly applied to tanks in which other liquefied gases are stored or transported, and are within the scope of the invention.
LPG is widely used for heating, cooking, agricultural applications, and air conditioning, especially in locations that do not have natural gas hookups available. In some remote locations, LPG is even used to power generators for electricity. LPG is typically held in pressurized tanks that are located outdoors and above ground. Under one atmosphere of pressure, the saturation temperature of LPG, i.e., the temperature at which it boils, is around −40° C. As pressure increases, so too does the saturation temperature. LPG is held in a liquid state by gas pressure inside the tank. As gas vapor is drawn off from the tank for use, the pressure in the tank drops, allowing more of the liquefied gas to boil to vapor, which increases or maintains pressure in the tank.
As the gas boils, the phase change from liquid to gas draws thermal energy from the remaining liquid, which tends to reduce the temperature of the LPG in the tank. If LPG temperature drops, the boiling slows or stops, as the LPG temperature approaches the saturation temperature. Thus, boiling LPG tends to increase pressure and saturation temperature, while at the same time tending to decrease the actual temperature of the LPG in the tank, until an equilibrium temperature is reached, at which the saturation temperature is equal to the current temperature of the LPG. Provided the energy expended to vaporize the gas does not exceed the thermal energy absorbed by the tank externally, from, for example, sunlight and the surrounding air, the LPG will continue to boil as vapor is drawn off, until the tank is empty. On the other hand, if more energy is expended to vaporize the gas than is replaced by external sources, the temperature in the tank will drop toward the equilibrium temperature, resulting in less energetic boiling, and a drop in tank pressure. If tank pressure drops too low, it can interfere with the operation of appliances and equipment that draw gas for use, such as furnaces, ovens, ranges, etc.
For purposes of the following disclosure, the maximum continuous rate at which gas can flow from a supply tank using only ambient energy to vaporize the LPG, without causing the tank pressure to drop below an acceptable level, will be referred to as the maximum unassisted flow rate. It will be recognized that this rate will vary according to the ambient temperature near the tank.
Low tank pressure is a particular concern in regions where ambient temperature can drop to very low levels, such as during the winter at high latitudes, or at very high altitudes. For example, when ambient temperature drops very low, the heat energy available to warm an LPG storage tank is reduced, while at the same time, the cold temperature prompts an increased draw of gas to fuel furnaces to warm homes and other buildings. As gas pressure drops below the regulated pressure of the gas line, flames in furnaces, water heaters, and other gas consuming appliances reduce in size, producing less heat and prompting users to open gas valves further, which only accelerates the pressure drop. Eventually, tank temperature can drop below the boiling point of unpressurized gas, at which point, no gas will flow. It can be seen that, as ambient temperature drops, the potential for unacceptable loss of pressure increases, as does the potential demand for gas, such as for heating.
To prevent such a pressure reduction, there are a number of measures that can be taken, which fall into three general categories, each with its own advantages and disadvantages.
In the first category, LPG is drawn from the bottom of a tank as a liquid, and passed through a separate vaporizer in the supply line, to meet demand. The volume of liquid flow has relatively little effect on tank—or system—pressure, because the liquid in the tank boils only to the extent necessary to replace the volume of fluid drawn from the tank. Thus, the limiting factor is more frequently the capacity of the vaporizer. In some limited situations, where, for example, the ambient temperature is very low, and the draw by the load is very high, tank pressure can still drop. In such cases, a vapor return line is frequently employed from the outlet of the vaporizer to the tank to increase the tank pressure.
There are a number of types of LPG vaporizers, including direct gas-fired and electrically heated. Some electric vaporizers with explosion-proof electrical connections can be mounted on or near the storage tank. However, safety regulations in most jurisdictions require that sources of combustion, such as an open flame, or heat sources that exceed the auto-ignition temperature of LPG, cannot be located in a same enclosure with an LPG storage tank, or within some minimum distance. Thus, a gas fired vaporizer must be positioned away from the storage tank, which adds cost and complexity, and increases maintenance requirements. Nevertheless, gas-fired vaporizers are more commonly used with large LPG storage systems, because the heating cost is generally lower than with electrically heated vaporizers. Additionally, gas-fired units can be used in locations where electricity is unavailable. A disadvantage of in-line vaporizers in general is that because they draw liquid from the bottom of the tank, they are always in operation, even when the maximum unassisted flow rate exceeds the current vapor demand.
In a second system configuration, gas for normal use is drawn from the top of the tank, but when pressure drops below a threshold, liquid is drawn from the bottom and boiled to vapor in a vaporizer and returned to the top of the tank to re-pressurize the tank. On one hand, such systems have more complex control, plumbing, vapor, and fluid circuits. On the other hand, these systems employ the vaporizer only when tank pressure drops below the threshold, so they tend to be more fuel efficient than in-line vaporizer systems.
In a third configuration, a tank heater is activated to warm the tank and its contents when tank temperature or pressure drops below a threshold. One type of tank heater comprises an electric element strapped to the tank. In another type, indirect heat is used, in which a medium, such as water or steam, is heated at a remote location, then piped to a heat exchanger in contact with the tank walls. Indirect heat is advantageous in situations where waste heat is available, such as where water is used to cool industrial machinery, etc.
Generally, disadvantages of many of the systems available are often related to the difficulty of providing heat in the close vicinity of an LPG tank without creating a condition that would be dangerous in the event of a tank leak or tank over-pressure. The complexity of systems in which a heat source is remotely located not only increases the cost, but also the likelihood of malfunction. Additionally, vaporizers and heaters that employ electric heating elements, or that are electrically controlled, are impractical for use in applications where electrical power is not available. In such cases, an electric generator is required to provide the electricity, resulting in costly efficiency losses.
One problem associated with electric tank heaters, in particular, is that the heating element is in direct contact with the tank wall. Temperature differentials between the element and the tank can promote water condensation, which can be trapped between the heating element and the surface of the tank, resulting in deterioration of the paint and subsequent corrosion of the steel tank wall.
Most jurisdictions have stringent regulations regarding the use of combustion sources near LPG tanks and gas transmission lines. These regulations dictate explosion-proof requirements for electrical connections, minimum distances to open flames, etc. The restrictions vary according to the size of a tank and proximity to public areas.